The present invention relates to a system for determining the phase and magnitude of an incident signal relative to a cyclical reference signal, and more specifically to such systems embodied in oscilloscope systems, and even more specifically to such systems embodied in digital storage oscilloscopes.
It has long been required that network parameters be measured, such as magnitude and phase of the response of circuitry to a reference signal, over a range of frequencies. Other applications require similar measurements.
Many methods have been developed for making such measurements. Early solutions used slotted waveguides with inserted detectors to measure standing waves, then the circuit responses were measured manually and plotted on a Smith chart. This method required substantial manual adjustments to the mechanism, measurements of signal characteristics, and mathematical calculations to produce a Smith chart over a range of frequencies. Later developments included instruments which could display a trace on an x-y display device, such as a cathode ray tube (CRT) in an oscilloscope, which, when a Smith chart was overlaid atop the CRT, provides a real-time plot on a Smith chart. However, these instruments generally operated in conjunction with variable frequency signal generators, and included synchronous detectors. Consequently, to take measurements over a range of frequencies, the synchronous detectors, and in particular the filters associated with those detectors had to be readjusted for each change in frequency.
More recently, network analyzers have been developed which can calculate network parameters over a range of frequencies with less manual work and more accuracy than the previous methods. However, network analyzers are very expensive pieces of test equipment, and have a limited range of use, i.e. measurement of network parameters.
In accordance with principles of the present invention, a system for determining the phase and magnitude of an incident signal relative to a cyclical reference signal, includes a sampler for sampling the incident signal. Autoranging circuitry is responsive to the reference signal and determines the frequency f of the reference signal and sets the sampling rate sr of the sampler. A processor, responsive to the sampler, computes a single point DFT v of the sampled incident signal responsive to the frequency of the reference signal and determines the phase e and magnitude |v| of the incident signal relative to the reference signal in response to the single point DFT.
A system in accordance with principles of the present invention provides the functions of the prior art systems which included a synchronous detector, but allows continuous operation of the system over a wide frequency range without requiring readjustment of any system component, such as filters.